This invention relates to liquid crystals, and, more particularly, to a substrate that pre-aligns a liquid crystal layer.
One type of liquid crystal electro-optical device is constructed by placing a thin layer of liquid crystal between two transparent plates that have each been coated with a layer of an electrical conductor on its interior face. When no electric field is applied between the conductive layers, the director of the liquid crystal is in one state. (The "director" of the liquid crystal is the macroscopic direction of the long molecular axis of the liquid crystal molecules.) When an electric field is applied, the director reorients to another state. Because the liquid crystal is birefringent, the two states will have different refractive indices, and in many cases the thin layer of liquid crystal will be birefringent for light directed against the transparent plates. The state change responsive to an applied electric field is the basis for liquid crystal devices that control light, such as displays and projectors.
In its usual form, a liquid crystal light valve is a device that modulates a polarized projection light beam directed against one side of the light valve according to a writing light beam directed against the other side. The polarized projection light beam enters the light valve through one of the transparent electrodes, usually termed the counterelectrode, passes through the liquid crystal layer, and is reflected from a mirror on the other electrode. The projection light beam passes back through the liquid crystal layer and the counterelectrode, and through an external analyzer. The polarizing and analyzing functions can often be accomplished by a single polarizing beam splitter through which the projection light beam passes before and after passing through the liquid crystal. The operation of this and other types of liquid crystal light valves is discussed in greater detail in numerous technical publications, see for example "Progress in Liquid Crystal Light Valves", by W. P. Bleha, in Laser Focus/Electro-Optics, October 1988, pages 111-120.
In this type of liquid crystal light valve, an electric bias field is applied through the liquid crystal layer by a voltage on the conductive electrodes. The liquid crystal is oriented responsive to this electric bias field. The director of the liquid crystal is initially, in the electric field-off state, aligned perpendicular to the plates. Application of the electric field causes the director to rotate toward the plane of the plate, changing the light transmittance of the device. The writing light pattern modulates the electric field, changing the phase retardation of the light passing through the liquid crystal, which in turn modulates the projection light beam passing through the analyzer.
One problem experienced with early liquid crystal light valves was nonuniformity of the projection light modulation across the face of the liquid crystal light valve. If the director of the liquid crystal is initially perfectly perpendicular to the plane of the plates, then application of the electric field causes the director to rotate into random azimuthal positions from place to place within the cell. The response of the cell to the writing light, and thence the transmission of the cell to the projection light, therefore varies with location within the cell, an undesirable result.
A satisfactory solution to this problem is to provide a surface that aligns the director of the liquid crystal with a slight pretilt that is uniform across the liquid crystal cell. That is, if the director is initially tilted several degrees (usually about 1.5 to about 2.5 degrees) from perfectly perpendicular, all in the same azimuthal direction, the application of an electrical field causes the director to further tilt uniformly in the same direction, until it lies in the plane of the plate and everywhere in the cell points in the same direction (or azimuth).
This solution has been known for a number of years, and a number of different approaches have been proposed for providing controlled pretilt to the liquid crystal director. Some of the approaches involve nothing more than rubbing the surface of the plate with a cloth in a single direction. Others involve treating the plates with multiple steps, depositions, and treatments of the surface. These more complex approaches variously suffer from shortcomings in temperature sensitivity, sensitivity to liquid crystal decomposition products, variation in the tilt direction or the magnitude of the tilt angle, and complexity and cost.
There remains a need for an effective, relatively inexpensive approach for treating plates to induce a uniform controlled tilt in a subsequently applied liquid crystal layer. The present invention fulfills this need, and further provides related advantages.